1. Field of the Invention
This invention relates to a satellite broadcasting system for providing broadcasting of images, voices, data, etc., using a broadcasting satellite located in a stationary orbit above the equator, and also to the broadcasting satellite used for the satellite broadcasting.
2. Discussion of the Background
In accordance with the demand for various kinds of broadcasting services and with the development of communication techniques, satellite broadcasting has been started in addition to ground broadcasting. Since satellite broadcasting can provide services in a wide area without establishing a large-scale infrastructure, attention is now paid to it as a medium that can satisfy many needs.
At present in Japan, BS (Broadcasting Satellite) broadcasting and CS (Communication Satellite) broadcasting are now put to practical use, and the use of digital broadcasting has been started. Also in foreign countries, a satellite broadcasting system of substantially the same scale as in Japan has been developed and put to practical use.
In the satellite broadcasting systems used in both Japan and foreign countries, certain transmitting stations collect broadcasting signals of a plurality of channels and multiplex them before transmitting them to an up transmission line leading to the broadcasting satellite. The broadcasting satellite, in turn, converts the frequency of each signal input thereto via the up transmission line, to a value corresponding to a down transmission line to the ground, and performs power amplification of each signal, thereby transmitting each resultant signal to a predetermined service area. Thus, all broadcasting signals of a plurality of channels are transmitted to the predetermined service area. In other words, broadcasting is always performed only for a single service area.
Furthermore, a feeder link station is necessary for collecting broadcasting signals of a plurality of channels, and then transmitting them to the up transmission line. In the case of collecting broadcasting signals from a broadcasting station in a real-time manner, it is necessary to secure a ground transmission line between the broadcasting station and the feeder link station, which inevitably makes the system complicated.
As stated above, in the conventional satellite broadcasting system, a particular feeder link station concentrically performs signal transmission to a broadcasting satellite, and the broadcasting satellite itself only relays signals transmitted from the feeder link station. Accordingly, the system structure is complicated, and the service area is disadvantageously inflexible.
The present invention has been developed in light of the above, and is aimed at providing a satellite communication system of a simple structure capable of changing the service area for each of broadcasting signals, and also providing a broadcasting satellite that enables the satellite communication system.
To attain the aim, the invention provides a satellite broadcasting system that uses a broadcasting satellite located in a stationary orbit above the equator to provide a user with broadcasting from a plurality of transmitting stations such as broadcasting stations, characterized in that the broadcasting satellite comprises: a reception antenna such as a Ka-band antenna for receiving a signal transmitted via a predetermined up transmission line on which a plurality of communication channels are set; a transmission antenna such as an S-band antenna having a plurality of radiators for emitting transmission beams to a plurality of areas; channel separating means such as a band-pass filter for extracting, from the signal received by the reception antenna, a signal transmitted through each of the plurality of communication channels; and a plurality of signal processing means such as synthesizers, high power amplifiers, etc. provided corresponding to the plurality of radiators for each multiplexing and amplifying predetermined signals included in signals transmitted through the communication channels and extracted by the channel separating means, thereby outputting each signal to a corresponding one of the radiators, and characterized in that each of the plurality of transmitting stations transmits a broadcasting signal to the up transmission line via the one of the plurality of communication channels assigned to the each transmitting station, which corresponds to a desired broadcasting area.
By employing the above-described means, a signal transmitted to the broadcasting satellite via the up transmission line is received by the reception antenna and separated by the channel separating means into signals corresponding to communication channels that are set on the up transmission line. Those of the thus-separated signals corresponding to the communication channels, which correspond to predetermined communication channels, are multiplexed and amplified by signal processing means that correspond to a plurality of radiators incorporated in the transmitting antenna, and are then output to the radiators corresponding to the signal processing means. The radiators, in turn, emit the signals to areas defined by the transmission beams of the radiators. Accordingly, a broadcasting signal transmitted from each transmitting station is transmitted from a radiator corresponding to a communication channel which was used to transmit the broadcasting signal, to an area defined by the transmission beam of the radiator. In other words, the transmitting station can determine a broadcasting area by selecting a communication channel to be used to transmit a broadcasting signal.
Further, in the invention, the plurality of communication channels include communication channels that correspond to the respective radiators, and a communication channel corresponding to a plurality of ones of the radiators. When there exist a plurality of signals transmitted through a plurality of communication channels and corresponding to radiators, corresponding signal processing means multiplexes the signals.
By employing the above means, a signal transmitted to the broadcasting satellite through a communication channel that corresponds to a plurality of ones of the radiators is transmitted to a plurality of areas from the radiators corresponding to the communication channel. Thus, the broadcasting area is not limited to the area defined by the transmission beam of each radiator, but an area that is obtained by combining areas defined by the transmission beams of a plurality of radiators can be used as one broadcasting area unit, thereby enhancing the flexibility of use of the communication system.
In addition, in the invention, each of the transmitting stations constructed as above transmits a broadcasting signal obtained by spread spectrum modulation. Further, when a plurality of signals are transmitted to each radiator through corresponding communication channels, the signal processing means multiplexes the signals by synthesizing them.
Since signals obtained by spread spectrum modulation do not interfere with each other, synthesization performed by the signal processing means is relatively simple processing. That the signal processing means performs simple processing implies that the system structure can be simplified.
FIG. 1 is a schematic view illustrating a satellite broadcasting system according to the embodiment of the invention;
FIG. 2 is a perspective view illustrating the outward structure of a broadcasting satellite SAT appearing in FIG. 1;
FIG. 3 is a view showing an example of a beam arrangement assumed when a service area is divided into four portions;
FIG. 4 is a block diagram illustrating the structure of a signal processing device installed in a satellite main body 1 appearing in FIG. 2;
FIG. 5 is a view showing an example of the relationship between a signal transmitted to the broadcasting satellite SAT via an up transmission line, and a signal transmitted from the broadcasting satellite SAT;
FIG. 6 is a view showing another example of the relationship between a signal transmitted to the broadcasting satellite SAT via the up transmission line, and a signal transmitted from the broadcasting satellite SAT;
FIG. 7 is a view showing a further example of the relationship between a signal transmitted to the broadcasting satellite SAT via the up transmission line, and a signal transmitted from the broadcasting satellite SAT; and
FIG. 8 is a view showing a yet another example of the relationship between a signal transmitted to the broadcasting satellite SAT via the up transmission line, and a signal transmitted from the broadcasting satellite SAT.